Method of fabricating an elongated artefact

ABSTRACT

A component 10 for making A15 Nb 3  Sn superconducting wire is of plane-filling cross-section after removing temporary additions 6, 7. It consists of a central pillar 1 of aluminium (later replaced by tin) surrounded by a two-deep array of polygonal copper columns 2/2a containing niobium rods. Many (e.g. 61) components 10 are voidlessly stacked together and extruded. The niobium rods adopt and retain a uniform distribution with minimum intervening material. This &#34;sixty-one&#34; member retains its shape during the extrusion and is itself of plane-filling cross-section. Several of them are voidlessly stacked together and on heat-treatment of the whole, the tin diffuses over a relatively short path and hence consistently into the rods, whereby there is formed a kilofilament Nb 3  Sn wire.

This invention relates to a method of fabricating an elongated artefactwherein the artefact comprises a matrix containing spaced parallelfilaments along the direction of elongation.

An example of such an artefact is a superconducting wire. Thesuperconducting component should not be thick normal to thecurrent-carrying direction, i.e. it should be merely a filament,otherwise magnetic fields will set up wasteful eddy currents in thecomponent. At the same time, a mere filament would be able to carry onlya small current, and therefore a superconducting wire conventionallyconsists of many parallel non-touching filaments of superconductorembedded in a matrix which is conveniently an ohmic conductor such asbronze or copper.

The theoretical potential of for example A15 superconductors such as Nb₃Sn has been known since 1960, but due mainly to their brittleness, inthirty years no ideal way of mass-producing them into wires has beenfound. Contributing to the difficulty is the requirement for the wiresto include a continuous phase of pure copper, to act as a normalelectrical conductor, heat sink and mechanical support in case the Nb₃Sn is accidentally warmed above its superconducting range.

Most conventional ways rely on forming some precursor of thesuperconductor to the final required shape, then converting theprecursor. For example, in the so called bronze route, rods of pureniobium are drawn down in a tin bronze to the extent that a fine wire isproduced with filaments of niobium embedded in it. This precursor isthen heated such that the niobium filaments are largely converted toniobium tin by reaction with the tin in the bronze. The maindisadvantage of this route is that if there is more than 13% tin in thebronze it becomes progressively brittle during drawing until It finallybreaks. This means that the mean current density in the final conductoris much reduced by the large volume of bronze required. (For clarityhereafter, the niobium is referred to as "filaments" even when thickenough, as at the outset, to count as "rods".)

The so called internal tin route attempts to avoid the requirement forthis large volume of bronze by including the tin separately in theprecursor in the form of pillars which are usually more than two ordersof magnitude larger than the niobium filaments. There can be problemswith drawing down even such precursors (the tin melts) and, as disclosedin UK Patent Application GB 2201830A, this can be partly mitigated byusing aluminium in place of tin; at a later stage in the method, whenthe cross-section of the composite has been substantially reduced bydrawing down or extrusion, the aluminium is removed from the compositeand replaced with tin.

The composite (conventionally of circular cross-section) is extruded ordrawn down, then an array of extruded composites is bundled together andfurther extruded, and so on, with as many of these stages as necessary.To avoid rupture of niobium filaments during the first extrusion, arelatively stout outer layer of copper is often left around thefilaments. It will be seen that this leads to opposing designconsiderations. At each of these stages of bundling, these stout layersof copper or subsequent copper extrusion cans become part of the volumeof the final conductor. Since copper competes with niobium for tin, thiswastefully increases the amount of tin which must be provided and alsoincreases the volume proportion of non-superconducting material. Tominimise this effect, the number of extrusion/bundling stages can bereduced. This entails cramming many niobium filaments into each singlestarting composite while, for reasons of manufacturing practicability,the tin remains present in one rather thick pillar per composite. Onheat treatment to react the niobium filaments with the tin, exchange ofcopper and tin between the regions thereof occurs via relatively longtortuous diffusion paths through the stack of filaments and predisposestowards the formation of Kirkendahl voids caused by the different ratesof diffusion of copper and tin. This is self-evidently a waste ofpotential current-carrying volume. Any design is a compromise betweenthese two effects.

The earlier-mentioned pure copper which is required is provided byenclosing arrays of composites inside barrier material such as tantalum,of thickness adequate to retain its integrity through extrusion, andencasing the whole in pure copper.

According to the present invention, there is provided a method offabricating an elongated artefact comprising a matrix containing spacedparallel filaments along the direction of elongation, the methodcomprising voidlessly stacking filaments each encased in a tube ofmatrix material to form an assembly having a space-filling shape,applying filler strips and a reduction can, reducing the cannedassembly, removing the can and the filler strips, voidlessly stacking aplurality of the reduced assemblies and reducing that stacked plurality.

The reduction may be by extrusion or drawing.

To the stacked plurality of the reduced assemblies, there may be appliedfiller strips and a reduction can before it is reduced. The then reducedstacked plurality may be chopped into lengths and the lengths thenvoidlessly stacked and then that voidless stack may itself be reduced.

The assembly may comprise, in addition to the filaments encased intubes, a pillar of another material such as tin. gallium, germanium oraluminium or a removable extrudable precursor. The filaments maycomprise niobium or other superconductor precursor or superconductor,and the matrix may comprise copper.

The invention extends to an elongated artefact made as set forth above.The said assembly may be a cylindrical component for use in fabricatingsuperconducting wire, comprising a central pillar of a stanniferous,galliferous and/or germaniferous material or of an extrudable removableprecursor thereof, or of aluminiferous material, preferably tin,surrounded by a two-deep array of cupriferous tubes each containing aniobiferous filament, at least the outer set of said columns beingpolygonal, and the cross-section of the component being a plane-fillingshape, whether before or after extrusion. By "cylindrical" it is clearwe mean the word In its topological sense, not the layman's sense of"right-circular cylinder", since the cylindrical component according tothe illustrated example of the invention has a polygonal exterior. Anadvantage of the two-deep array is the shortening of thegallium/germanium/tin diffusion pathway from the central pillar to itsmost distant filament compared with GB 2201830 A, thus yielding a moreuniform tin concentration in the product. The niobiferous metal maycontain for example titanium and/or tantalum additives, which increasethe upper critical field of Nb-Sn, e.g. Ti and/or Ta in quantities of upto 10% by weight.

As the cross-section of the component is a plane-filling shape, i.e.,repeated indefinitely in the same size, an unlimited number of thecomponents can be close-packed to fill a plane without voids. (Regularhexagons and squares are examples of plane-filling shapes, but aplane-filling component according to the invention would normally bemore complex in shape.)

The component is preferably further surrounded by removable fillerstrips of an extrudable metal or alloy with a higher melting point thanany one of tin, germanium or gallium, so profiled as to impart to saidcomponent a void-free extrudable cross-section, such as regular hexagonor a circle. Since presses capable of (the theoretically more ideal)hexagonal-to-hexagonal hydrostatic extrusion number well under one percontinent, it is alternatively possible to make the componenttemporarily right-circular-cylindrical (using the removable fillerstrips) to widen the choice of providers of extrusion services thefiller strips and any surrounding extrusion can being removed after theextrusion.

The invention extends to an Intermediate member comprising aclose-packed array of the components made as set forth above (any ofsaid removable filler strips having been removed). Because the saidcomponents are not surrounded by the previously necessary stout outerlayer of copper, not only is tin saved and the volume more efficientlyused for carrying current, but the spacing of the said tubes in theintermediate member is substantially constant even across the joinbetween adjacent components, thus assisting uniformity of propertiesafter heat-treatment (described later), reducing the risk of Kirkendahlvoids and reducing the risk that when the niobium filaments are expandedby absorbing tin, neighbouring superconductor filaments fromneighbouring arrays will come into contact, permitting wasteful eddycurrents laterally to the length of the filaments.

The invention further extends to a method of fabricating asuperconducting wire, comprising applying external filler strips to thesaid intermediate member, these strips being so profiled as to impart tothe member a substantially void-free extrudable cross-section, whichitself is preferably plane-filling, such as a regular hexagon, and maybe surrounded by a diffusion barrier such as tantalum foil, optionallywith an exterior niobium layer. The member (preferably then encased inan extrusion can) may then be worked (e.g. extruded or drawn) into theshape of a wire.

At some stage in the above, the central pillars may be removed (e.g.melted or dissolved out, for example if of aluminium, dissolved out byhot sodium hydroxide) and replaced by stanniferous metal, or aluminiummay be left. Then the member may be heat-treated to diffuse the tin oraluminium across the tubes into the filaments, to form the Nb₃ Sn or Nb₃Al superconductor in the form of spaced parallel filaments along thelength of the member. The tubes effectively form a substantiallycontinuous matrix, usually comprising copper.

The invention will now be described by way of example with reference tothe accompanying drawings, in which

FIG. 1 is cross-section of a cylindrical component according to theinvention, roughly full-size,

FIG. 1A is a cross-section of an alternative design to FIG. 1,

FIG. 2 is a cross-section of an intermediate member according to theinvention, also roughly full-size,

FIG. 3 is a cross-section of an assembly of several of the FIG. 2intermediate members, and

FIG. 4 is a cross-section of a special intermediate, shown enlargedabout tenfold (linear magnification), used in the preparation ofexternal filler strips for the intermediate member.

Turning to FIGS. 1 and 1A (which are alternatives), a cylindricalcomponent 10 comprises a central duodecagonal pillar 1 of aluminiumsurrounded by a two-deep array of polygonal copper tubes 2 eachcontaining a rod or filament of niobium. Of the thirty tubes 2,twentyfour will be seen to be regular hexagons, the rest 2a being of aspecific pentagonal shape (nearly as easy to make both as regards copperand niobium) to fill the shape. (It could be envisaged for the six 2aand the six others on the inner ring to have an arcuate inner edge, toencircle a circular cross-sectional pillar 1. Other variations are alsopossible. However, the layout in the Figures represents an optimalvolume ratio of aluminium to niobium.) Temporarily, the component 10 issurrounded by aluminium filler strips 6 encased in a strippable copperprotective sheath 7, the whole being substantially void-free and readilyextrudable. The whole is preheated to 200° C. to promote bonding of thestructure.

The whole is extruded to one-thirtieth of the starting cross-sectionalarea, maintaining the hexagonal (FIG. 1) or circular (FIG. 1A)cross-section, whereby internal compression is isotropic and the shape(despite the thirtyfold reduction) is not disturbed at all. Much work isdone, and hence heat is generated, during this operation, and thetemperature rises to a level which would have melted tin but does notmelt the aluminium. The heat usefully bonds the copper tubes 2 together.

The copper sheath 7 is stripped off and the aluminium filler strips 6are removed by dissolution in caustic soda. The (reduced) component 10is assembled in close-packed (void-free) array with sixty more in agenerally hexagonal array to form an Intermediate Member, indicated as"20" in FIG. 2.

There are now several choices of route to the desired superconductorwire. Five examples will be described.

Route 1. For this route, it may be convenient to go directly to a largerhexagonal array of components 10, the next larger size containingninety-one of them, and the next size again containing 127.

The Intermediate Member (the array of sixty-one (or 91 or 127)components 10) is then surrounded (In the "sixty-one" version as shownin FIG. 2) by twentyfour filler strips 21 and six corner filler strips22 to present a regular hexagonal exterior. This is wrapped in tantalumfoil 23, which acts as a tin diffusion barrier. In this and thealternative Routes, the Intermediate Member wrapped in tantalum foil 23may then be wrapped in niobium foil, not shown. (The filler strips 21and 22 are described in more detail later.)

Let this be Stage A. Then arcuate filler strips of copper are appliedaround the tantalum foil 23 (or of course the niobium foil if present),the copper strips being so profiled as voidlessly to encase the foil 23in a right-circular cylinder. This is inserted into a copper extrusioncan, the copper being a necessary part of the final product as explainedabove, and the whole drawn to the final wire size. Let this be Stage B.

Route 2. The Intermediate Member (the array of sixty-one components 10),item 20 of FIG. 2, is surrounded by twenty-four filler strips 21 and sixcorner filler strips 22 to present a regular hexagonal exterior. This iswrapped in tantalum foil 23, which acts as a tin diffusion barrier. (Thefiller strips 21 and 22 are described in more detail later.) A thickertantalum can may be expedient in some cases, instead. Let this be StageA. This is extruded down to one-tenth of its starting area. Let that beStage B. The tantalum-clad extrusion-reduced Intermediate Member isinserted into a close-fitting hexagonal copper tube, and seven (ornineteen, thirty-seven, sixty-one . . . ) of the tubes are assembledinto a close-packed hexagonal array. Aluminium arcuate filler strips areapplied to the outside of this array, so profiled as voidlessly toencase the array in a hexagon or right-circular cylinder as convenient;this is inserted into a copper extrusion can and extruded and/or drawnto the final wire size. The copper can may then be removed (bydissolution in nitric acid), and then the aluminium (by dissolution incaustic soda).

Route 3. This is identical to Route 2 except for a modification in casethere is no access to a hexagonal-to-hexagonal extrusion press as isnecessary immediately after Stage A. in Route 3, Stage A is followed byapplying arcuate aluminium filler strips to the outside of this array,so profiled as voidlessly to encase it in a right circular cylinder,which is canned in copper. This is subjected to circular→circularextrusion to one-tenth of its starting area. The copper is then removedby dissolution in nitric acid, followed by the aluminium (dissolved incaustic soda). This is Stage B, and Route 2 is rejoined at that point.Route 4. The Intermediate Member (the array of sixty-one components 10),item 20 of FIG. 2, is surrounded by twenty-four filler strips 21 and sixcorner filler strips 22 to present a regular hexagonal exterior. This iswrapped in aluminium foil and then inserted into a hexagonal copperextrusion can, the aluminium serving as a copper-copper antibondinglayer. This is Stage A. The whole is extruded to one-tenth of itsstarting area. This is Stage B. The copper can is dissolved away bydissolution in nitric acid and the aluminium foil is dissolved away bydissolution in caustic soda. The resultant reduced Intermediate Memberhas a space-filling cross section, and seven (or 19 or 37 . . . ) ofthem are voidlessly stacked in hexagonal array. That array is wrapped intantalum foil (to act as a tin diffusion barrier) and arcuate copperfillers are applied round it, so profiled as voidlessly to encase thefoil in a right-circular cylinder. This is inserted into a copperextrusion can, the copper being a necessary part of the final product asexplained above, and the whole drawn to the final wire size.

Route 5. The intention is to assemble seven Intermediate Members 20 inhexagonal array, as shown in FIG. 3, to form the final superconductingwire. These Members are notionally labelled 20¹, 20² . . . 20⁷,according to their intended individual positions in the hexagonal array.Member 20¹ is made into a regular hexagon by adding filler strips -21and -22 as explained in FIG. 4 later. It is wrapped in aluminium foil(to serve as a copper-copper antibonding layer) and Inserted into ahexagonal copper extrusion can. Members 20² -20⁷, which are in factidentical, are each made into a regular hexagon by adding the fillerstrips -21 and -22 to three adjacent sides (those which will abut otherMembers 20) and adding strips +21 and +22 to the remaining (open) threesides. The strips -21 and -22 have the same shape as their counterparts+21 and +22 but are of aluminium. (+21 and +22 are identical to 21 and22 of FIG. 4.) Then the Members 20² -20⁷ are each wrapped in aluminiumfoil and inserted into a hexagonal copper extrusion can. This is StageA. Then all seven Members are separately extruded to one-tenth of theirarea. That is Stage B. The copper extrusion can is dissolved away usingnitric acid, and the aluminium (foil, and strips -21 and -22) isdissolved away using caustic soda. The seven Members 20¹ -20⁷ can now bevoidlessly stacked as originally envisaged in FIG. 3. It will beobserved that they cannot in fact be assembled in any other than thecorrect orientations. That stack is is wrapped in tantalum foil (to actas a tin diffusion barrier) and arcuate copper fillers are applied roundit, so profiled as voidlessly to encase the foil in a right-circularcylinder. This is inserted into a copper extrusion can, the copper beinga necessary part of the final product as explained above, and the wholedrawn to the final wire size. That final drawing, If started at 77K,allows quite a respectable reduction such as to 1/10 of area withoutexceeding an output temperature of 200C. In that way, the tin (explainedin a moment) is not melted.

At either Stage A or B of any Route, the aluminium pillars 1 aredissolved out using hot sodium hydroxide. Stage A is preferable becausethat dissolution is easier but Stage B is also preferable because theA→B extrusion is easier with aluminium in the pillars 1 than with itsreplacement. The aluminium is replaced by solid tin pillars or by moltentin, which is caused to flow into the pillars 1.

The product of each Route may be bench-drawn then taken through wiredies as required, and the wire made into a winding as necessary for anelectrical machine. By this time the individual columns 2 are filamentsunder ten microns across. Nonetheless, they retain an excellentparallelness along the length of the product and an excellent regularityof spacing, even across the boundary from one component 10 or evenMember 20 to the next. The product is lightly twisted in use (e,g. 1turn per cm) in order to decouple the filaments electrically. Thoughhelical, the filaments remain parallel within the meaning of thisspecification.

Then (or at any time after the tin was introduced if there was to be nosubsequent strain greater than about 0.2% within the wire) the wire isheat-treated. The tin in the pillars 1 diffuses through the coppermatrix to the niobium (at no point having any great distance to go),forming in situ Nb₃ Sn (A15) superconductive kilofilament (butnon-touching) wires.

Turning to FIG. 4, a special intermediate is shown enlarged for clarity,made up of hexagonal and part-hexagonal columns, which are identical tothe tubes 2 of FIG. 1 in hexagon size, and some of which are furtheridentical in that they contain niobium filaments. The composition ofeach column is shown. Despite the apparent complexity of thepart-hexagonal columns, only three different pairs of part-hexagonaldies are needed altogether. The special intermediate is geometricallythe same as the component 10 of FIG. 1 and is extruded in the same way.

Then it is disassembled by etching in hot caustic soda (which removesthe aluminium) to yield the filler strips 21 and 22 (as labelled), whichwere mentioned above.

These filler strips serve to preserve the overall optimum compositionand, by filling the space, allow the subsequent extrusions to beperformed with no distortion of the niobium filaments, thereby at thesame time allowing a full use of the volume for conductors (not wastingit with voids or excess inert material) and minimising the incidence ofadjacent niobium filaments touching, which would allow wasteful eddycurrents lateral to the filaments; in other words our actual filamentdiameter should closely approximate to the "effective diameter", whichin the prior art is wastefully large because of touching filaments. Theproducts of Route 2 or 3 show a network of `veins` of tantalum and purecopper throughout their thickness. Although this is a loss of potentialsuperconducting volume, it improves the safety margin if there islocalised heating to above the superconducting temperature, by providinga nearby `relief pathway` for accepting current and removing excessheat. The products of Route 1, 4 or 5, on the other hand, have tantalumand pure copper on the outside only, such that--the total amount ofcopper being held the same--the volume proportion of tantalum is lessfor a given barrier thickness. This improves the current-carryingcapacity per unit cross-sectional area of the wire but reduces theelectrothermal stability of the wire.

We claim:
 1. A method of fabricating an elongated artefact comprising amatrix containing spaced parallel filaments along the direction ofelongation,the method comprising substantially voidlessly stackingfilaments each encased in a tube of matrix material to form an assemblyhaving a space-filling shape, applying filler strips and a reductioncan, reducing the canned assembly, removing the can and the fillerstrips, substantially voidlessly stacking a plurality of the reducedassemblies and reducing that stacked plurality.
 2. A method according toclaim 1, wherein the reduction is by extrusion.
 3. A method according toclaim 1, wherein the stacked plurality has filler strips and a reductioncan applied to it before it is reduced.
 4. A method according to any oneof the preceding claims, further comprising substantially voidlesslystacking the reduced pluralities and reducing that stack of pluralities.5. A method according to claim 1, wherein the assembly comprises, inaddition to said filaments encased in tubes, a pillar of anothermaterial.
 6. A method according to claim 5 wherein said pillar comprisestin, gallium, germanium.
 7. A method according to claim 1, wherein thematrix material comprises copper and the filaments comprise niobium. 8.A method according to claim 6, further comprising the steps of selectingthe materials for their suitability in fabricating superconducting wire,arranging the encased filaments so as to surround the pillar in atwo-deep array, at least the outer set of said matrix-materialencasements being polygonal, and reducing the said stacked plurality insuch a way as to form a plane-filling shape.
 9. A method according toclaim 8, wherein the filler strips comprise cupriferous tubes containingniobiferous filaments.
 10. A method according to claim 8, wherein themember is surrounded by a diffusion barrier.
 11. A method according toclaim 10, wherein the member is worked into the shape of a wire.
 12. Amethod according to claim 9, wherein the central pillars are of anextrudable removable precursor of a material selected from the groupconsisting of stanniferous, galliferous and germaniferous materials, andwherein the method further comprises the steps of removing and replacingsaid precursor with one of a stanniferous, galliferous and germaniferousmaterial, and heat-treating the member to diffuse the tin, gallium andgermanium, respectively, across the tubes into the filaments to form thesuperconductor.
 13. A method according to claim 9, wherein the centralpillars are of one of a stanniferous, galliferous, aluminiferous andgermaniferous material and wherein the method further comprisesheat-treating the member to diffuse the tin, gallium, aluminium andgermanium, respectively, across the tubes into the filaments to form thesuperconductor.